Albert, T. S. E., V. L. Tucker, and E. M. Renkin. Atrial natriuretic peptide levels and plasma volume contraction in acute alveolar hypoxia. J. Appl. Physiol. 82(1): 102-110, 1997.Arterial oxygen tensions (Pa_O2), atrial natriuretic peptide (ANP) concentrations, and circulating plasma volumes (PV) were measured in anesthetized rats ventilated with room air or 15, 10, or 8% O₂ (n = 5-7). After 10 min of ventilation, Pa_O2 values were 80 ± 3, 46 ± 1, 32 ± 1, and 35 ± 1 Torr and plasma immunoreactive ANP (irANP) levels were 211 ± 29, 229 ± 28, 911 ± 205, and 4,374 ± 961 pg/ml, respectively. At Pa_O2 40 Torr, irANP responses were more closely related to inspired O₂ (P = 0.014) than to Pa_O2 (P = 0.168). PV was 36.3 ± 0.5 µl/g in controls but 8.5 and 9.9% lower (P  0.05) for 10 and 8% O₂, respectively. Proportional increases in hematocrit were observed in animals with reduced PV; however, plasma protein concentrations were not different from control. Between 10 and 50 min of hypoxia, small increases (+40%) in irANP occurred in 15% O₂; however, there was no further change in PV, hematocrit, plasma protein, or irANP levels in the lower O₂ groups. Urine output tended to fall during hypoxia but was not significantly different among groups. These findings are compatible with a role for ANP in mediating PV contraction during acute alveolar hypoxia.